1. Field of the Invention
The present invention relates generally to systems and methods for display monitor control, and more particularly to a system and method for automatically calibrating display monitor beam currents.
2. Discussion of Background Art
Maintaining color accuracy in computer monitors is of increasing concern to many computer users as well computer designers and manufacturers. The proliferation of computers for use in applications where color accuracy is critical makes faithful color reproduction more than merely an aesthetically pleasing feature in a computer monitor. Fields where color accuracy may be critical may include, for example, medicine, computer graphics, and engineering design work.
Tristimulus values, as further explained in Color Measurement, Theme and Variation, D. L. MacAdam, 2nd ed., Springer-Verlag, pp. 9-21, represent the amount of light energy in overlapping bands referred to as X, Y, and Z. The X, Y, and Z bands correspond to the three channels of a model of human color vision known as the C.I.E. standard of 1976, in which average observers perceive specific hues according to the ratios of light energy in the three bands X, Y, and Z. The tristimulus X, Y, and Z value ratio corresponds to a particular hue. Further, the summed weighted energies of these three bands describe the intensity or luminance of the light. Thus, a given set of tristimulus values may be used to effectively represent a specific a hue at a specific luminance.
Various factors degrade the color of images displayed on computer monitors. A significant factor is cathode ray tube (CRT) aging. Over time, electron and ion bombardment changes the hue and luminous efficiency of the light emitted from the phosphors used in the face of a cathode ray tube. The mechanism of these changes is thought to be the generation of non-emitting recombination centers and/or the loss of activator centers due to changes in the state of ionization of activator constituents. Each of the three primary colors uses a respective phosphor with a different chemical composition (having a different rate of deterioration and aging) which also contributes to the total hue shift.
The rate of CRT color degradation depends primarily upon beam current, acceleration voltage, and CRT temperature. If the acceleration voltage and temperature are held constant, as is typical in CRT displays, then phosphor degradation is substantially a function of the accumulated number of Coulombs of beam current that have passed through the cathode onto the phosphors of the CRT.
Another significant contribution to color degradation is the aging of the CRT's glass faceplate. High-energy electron and X-ray bombardment alters the chemical structure of the faceplate glass and unevenly reduces its transmission of light, dramatically more at shorter wavelengths than at medium and longer wavelengths, thus shifting the transmission of hues toward yellow. The faceplate's rate of change for light transmission depends primarily upon the total amount beam current and acceleration voltage over time.
Color degradation is compensated for most commonly by adjusting the computer monitor's beam current. The beam current is adjusted indirectly by varying the gain of the computer monitor's video amplifiers. There is one video amplifier for each of the red, green, and blue electron guns within the monitor. Since the relationship between the gain of each video amplifier and the resultant beam current is non-linear, the beam current in most monitors is manually calibrated.
For manual calibration, the computer monitor is provided with individual manual color controls for adjusting the gain of its red, green, and blue video amplifiers as well as its overall luminance. A spectra-radiometer, or a photometer, is then used to objectively measure the monitor's tristimulus values using a white screen generated on the monitor's display. The spectra-radiometer measures and displays the tristimulus values of this image. Using these tristimulus values, the gain of the red, green, and blue video amplifiers are adjusted using the manual color controls. The color controls are adjusted until the tristimulus value readings on the spectra-radiometer match a corresponding set of tristimulus values of a desired chromaticity image.
Another manual calibration method involves comparing a test-pattern generated on the monitor with a series of colored cards. This system is relatively inaccurate since it relies on a subjective comparison between the test-pattern and the colored cards.
What is needed is a system and method for automatically calibrating display monitor beam currents that solves the foregoing problems of conventional manual calibration systems.
A system and method for automatic calibration should account for the fact that the relationship between the gain and current in a video display is unknown and changes over the life of the display. Such a system and method should also be relatively fast and relatively stable, even when faced with unstable or fluctuating beam current measurements.